ESR Study of Rotational Isomerism in Substituted Benzaldehyde Anions

Abstract

The ESR spectra of terephthalaldehyde-2,5-d2, phthalaldehyde, phthalaldehyde-4-d, o-nitrobenzaldehyde, and p-nitrobenzaldehyde anions have been obtained and analyzed. It is shown experimentally that the trans rotamer of the terephthalaldehyde anion is more stable and has a higher ratio of ring proton coupling constants than the cis rotamer. In each rotamer the largest coupling constant is due to the aldehyde protons. All detectable phthalaldehyde anions exist in the unsymmetrical meso conformation, in which one pair of coupling constants has been assigned experimentally to a pair of ring protons. The other two pairs are assigned theoretically. The values of all coupling constants in both terephthalaldehyde anion rotamers can be calculated very accurately by either of two crude extensions of simple MO theory, designated the β- and α-effects. These two effects predict opposite assignments of ring proton coupling constants within each rotamer, and the detailed assignment cannot be established by deuteration experiments. When the β and α effects are applied to phthalaldehyde anion, the results explain the observed coupling constants only qualitatively. Spin densities are predicted by MO calculations for benzaldehyde anion, which was not detected experimentally. The application of the superposition model to terephthalaldehyde anion is discussed. The large relative stability of the meso rotamer of the phthalaldehyde anion is attributed to an intramolecular hydrogen bond.